1. Field of the Invention
This invention relates to a light branching and insertion apparatus suitable for use, in an optical transmission system wherein optical communication is performed between a plurality of terminal stations, to branch optical transmission lines each formed from an optical fiber to interconnect the terminal stations, and also to an optical transmission system having a light branching and insertion apparatus of the type mentioned.
2. Description of the Related Art
In order to allow communication of signal light between terminal stations spaced from each other, for example, with a sea interposed therebetween, an optical transmission system has been developed wherein an optical fiber cable is laid on the bottom of the sea and signal light is repeated and amplified in a plurality of stages using a plurality of optical amplifiers provided for the optical fiber cable.
An example of such an optical transmission system laid on the bottom of a sea as just described will be described with reference to FIG. 15.
Referring to FIG. 15, the optical transmission system generally denoted at 100 is constructed to effect bidirectional communication between a plurality of terminal stations, and includes, as such plurality of terminal stations, for example, an A station 101, a B station 102, a C station 103 and a D station 104. The A to D stations 101 to 104 are connected to each other by optical transmission lines 107 each formed from an optical fiber via light branching and insertion apparatus 105 and 106.
In particular, as seen from FIG. 15, the A station 101 and the C station 103 are connected to each other with the light branching and insertion apparatus 105 interposed therebetween, and the B station 102 and the D station 104 are connected to each other with the light branching and insertion apparatus 106 interposed therebetween. The A station 101 and the B station 102, the A station 101 and the D station 104, the B station 102 and the C station 103, and the C station 103 and the D station 104, are connected to each other with the light branching and insertion apparatus 105 and 106 interposed therebetween.
The optical transmission lines 107 which connect the A station 101, B station 102 and C station 103 to each other in the light branching and insertion apparatus 105 shown in FIG. 15 are specifically shown in FIG. 16.
Referring to FIG. 16, the light branching and insertion apparatus 105 shown is interposed at branching points of the optical transmission lines 107 which connect the A station 101, B station 102 and C station 103 to each other, and the A station 101 and the B station 102, the A station 101 and the C station 103, and the C station 103 and the B station 102, are connected to each other with the light branching and insertion apparatus 105 interposed therebetween.
Referring back to FIG. 15, a plurality of optical amplifiers 108 for amplifying signal light to be transmitted in a plurality of stages are provided for each of the optical transmission lines 107.
In the optical transmission system 100 shown in FIG. 15 having the construction described above, signal light is communicated among the A to D stations 101 to 104.
However, since, in the light branching and insertion apparatus 105 shown in FIGS. 15 and 16 or the light branching and insertion apparatus 106 shown in FIG. 15, the optical transmission lines 107 overlap with each other between the different terminal stations, an optical transmission system which employs such a light branching and insertion apparatus as just described has a subject to be solved in that, if the number of terminal stations provided in the optical transmission system increases, the optical transmission lines and the light branching instruction apparatus are complicated in construction.
On the other hand, in recent years, research and development for an optical transmission line formed from an optical fiber for wavelength multiplexing (WDM) which effects optical wavelength multiplexing transmission and for an optical amplifier for WDM have been and are being proceeded. In an optical transmission system which employs such an optical transmission line or optical amplifier for WDM as just mentioned, signal light of a plurality of wavelengths can be transmitted by means of a single optical transmission line, and the signal light of the plurality of wavelengths transmitted in this manner can be amplified by a single optical amplifier.
Therefore, a demand for, as a light branching and insertion apparatus to be applied to an optical transmission system for WDM, a light branching and insertion apparatus which can effect, when signal light of a plurality of wavelengths is transmitted without using overlapping optical transmission lines, branching (wave demultiplexing) or insertion (wave multiplexing) of the signal light depending upon the wavelength is increasing.
One of such light branching and insertion apparatus is a fiber grating filter wherein gratings are formed on two buses of a Mach-Zehnder interferometer. In the fiber grating filter, the gratings are formed so as to reflect signal light of a desired wavelength in order to allow branching and insertion of signal light of the wavelength.
However, since a fiber grating filter having the construction described above is formed from an optical fiber, characteristics thereof are sometimes varied by a temperature variation. Further, since a branching element and an insertion element for signal light are formed from optical couplers, there is another subject to be solved in that the loss in branching and insertion of signal light is high.
Further, such a fiber grating filter as described above is applied to an optical transmission system which effects one-directional communication, and there is a further subject to be solved in that the fiber grating filter cannot be applied as it is to another optical transmission system which effects bidirectional communication.